Applications of LCP Materials

Polymeric Materials, Royal Institute of Technology, Stockholm, Sweden 

The industrial development of thermotropic liquid crystal polymer (LCP) materials can be traced from its theoretical origins, through the identification of useful compositions, to full commercialization. The future industrial challenge will be to define and develop applications which take advantage of the unique properties of these materials. 

Representative classes are found in wholly aromatic polyesters, aromatic-aliphatic polyesters, wholly aromatic polyesteramides, aromatic-aliphatic polyesteramides, aromatic polyazomethines, aromatic polyester-carbonates, etc. 

Thermotropic LCP materials have very good thermal, physical, dielectric, optical and mechanical properties as well as good chemical resistance, low flammability, very good dimensional stability, etc. In addition to their good product properties they show remarkable ease of processing thanks to their low melt viscosity and high melt strength. 

Already commercial demands are growing in the fibre-optic, chemical process, electric/electronic, aerospace, automotive and household markets and new areas of potential applications continue to emerge. 

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TLCPs have many outstanding properties that uniquely qualify them for these high performance multilayer boards (12). Table 7 compares the applicability of LCPs with other state-of-the-art materials for electronic packaging. They can be made into very thin, self-supporting films (<50 J.m) with a controllable CTE. By processing LCP films as described above, circuit substrates can be manufactured with a CTE around 7 ppm/°C and thermal stability over 250°C. TLCPs do not require secondary resins for fabrication into MLBs, they can be thermally bonded to themselves and to copper foil. Control of molecular orientation has been shown to result in a substrate with the desired CTE of 6 to 7 ppm/°C for matching alumina, or 16 ppm/°C for matching copper. 

The major applications of LCP films in electronics are multilayer boards and multichip modules, and flexible printed circuits. Current materials used in the.se applications include polyimide film, high performance thermoplastic film such as polyphenylene sulfide and polyetherether ketone, and fiber reinforced composites such as quartz fiber-polyimide. The price of these materials for electronic packaging is in the range of. 50 to. l(K)/lb, so LCPs can compete very effectively at their current prices. Based on a two to three times increase in price... 

In order to tap the commercial applications of LCP in packaging, processing must achieve efficient use of the high-performance polymer. There are many high value polymers and other materials that are used in thin layers. A prime example is ethylene vinyl alcohol (EVOH), a super barrier material used as a thin layer in coextruded film and multilayer containers such as ketchup bottles. Other examples are coatings, such as metal, glass, and polymeric coatings. LCP used in thin multilayer constructions should compete well with these other materials. 

Chapter 1 gives a classification of the various ways in which the meso-gens may be connected to the polymer chains. Currently, the bulk of LCP material is based on main chain or longitudinal LCPs for use in engineering applications. The side chain or comb polymers are intended for use in electronics and opto-electronic systems and as surfactants. Many other variants and possibilities exist but their properties have not yet been fully studied or used. In this respect it is hoped that the current work will indicate future possibilities as well as discussing current opinion. 

Chapters 2 and 3 describe methods of characterizing the mesophases. In the former a comprehensive guide to textures is given. Chapter 4 describes the dielectric properties of LCPs. Chapters 5 to 8 inclusive deal with comb and longitudinal LCPs, lyotropic and thermotropic. In these chapters are discussed the syntheses, characterization, structures and properties of these materials. Chapter 9 deals with some applications of LCPs. 

Regarding the possible applications of metathesis-derived LCPs, it was shown that the mesophases of SCLCPs can be aligned in magnetic fields, leading to optically transparent materials with high birefringence (Sect. 2.1). These materials are certainly interesting materials for optical applications. 

Although the technical applications of low molar mass liquid crystals (LC) and liquid crystalline polymers (LCP) are relatively recent developments, liquid crystalline behavior has been known since 1888 when Reinitzer (1) observed that cholesteryl benzoate melted to form a turbid melt that eventually cleared at a higher temperature. The term liquid crystal was coined by Lehmann (2) to describe these materials. The first reference to a polymeric mesophase was in 1937 when Bawden and Pirie (2) observed that above a critical concentration, a solution of tobacco mosaic virus formed two phases, one of which was bireffingent. A liquid crystalline phase for a solution of a synthetic polymer, poly(7-benzyl-L-glutamate), was reported by Elliot and Ambrose (4) in 1950. 

The unique molecular packing of rod-like chains in liquid crystalline polymers (LCP) closely resembles the extended chain structure of highly oriented flexible chain polymers, suggesting that these materials are good candidates for barrier applications. Thermotropic LCP s, first developed in the early 1970 s, have been the object of much interest because of their excellent mechanical properties and ease of product fabrication. Preliminary observations have shown that a commercially available wholly aromatic thermotropic copolyester has gas permeability coefficients that are lower than those of polyacrylonitrile (4.). These results raise some fundamental questions as to the nature of the mechanism for transport of small molecules through a matrix of ordered rigid rod-like chains. 

More recently,thin walled articles have been fabricated by blow-molding composites of liquid crystal polymer (LCP) and expanded porous polydetrafluoroethylene sheetingl l material. Container application examples include food and pharmaceuticals, automotive gas tanks, bottles, and other vessels. Unlike most other thermoplastic polymers, thermotropic LCP forms high-viscosity melts that have thixotropic characteristics. Applying shear force to the melt substantially alters the melt viscosity of LCP and the orientation of its polymer domains. These attributes are useful to... 

Completely new a ects came into this field with the development of LCPs. Generally, conventional non-mesomorphic polymers play an important role in many technological areas, as deduced from their countless applications [17]. In particular, by exploiting the variety in chemical structures polymeric materials with quite different macroscopic properties can be produced. Here, the realization of LCPs offers new and interesting scopes of tedmolo cal applications, based on the unique combination of specific polymer features with the anisotropic properties of the LC state [18-21]. 

LCP fibres are used for making heavy-duty ropes and belts and also as the reinforcing fibres of composite materials. An important application of Kevlar is in the fabrication of bullet-proof clothing. The easy mouldability, good mould filling and low thermal expansion of thermotropic LCPs, combined with their resistance to solvents and their stability at temperatures up to 120 °C or higher make them suitable for precision mouldings for use in a variety of components such as electrical connectors. 

The major advantage for the usage of LCPs as stationary phases for LC applications is that coating of the polymer on the silica gel is a simple process. However, also comb-shaped polymers prepared by octadecyl-acrylate and 3-mercaptopropyltrimethoxysilane as chain transfer agent can be immobilized on silica gel by bonding. It was shown that the telomer behaves as nematic material in the range of 42-47°C. The separation of geometrical isomers could be achieved. ... 

In recent years, nanotechnology and nanomaterials have been widely used in designing advanced functional materials based on photoresponsive LCPs. Although several reviews have concentrated on the photodeformable effect of LCPs and their applications in soft actuators [22], to date the influence of nanostructures and nanomaterials on the photodeformable properties of LCPs has not been summarized. In this chapter, we mainly focus on the utilization of special nanostructures and amazing physicochemical properties of nanomaterials to manipulate the photomechanical behaviors of LCPs. Fmthermore, their potential applications as light-driven devices and other future prospects are proposed. 

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